1. Field of the Invention
The present invention relates to an optical scanning apparatus in which a plurality of deflection mirror surfaces are used for scanning with a light beam, e.g., an optical scanning apparatus. In the apparatus, a light beam deflected by one deflection mirror surface is deflected further by another deflection mirror surface and the deflection angle of the light beam accordingly increases, and also to an image forming apparatus which comprises such an optical scanning apparatus.
2. Description of the Related Art
In an attempt to increase the deflection angle of a light beam, a conventional optical scanning apparatus for use within an image forming apparatus such as a laser beam printer, a copier machine and a facsimile machine in some cases employs a structure that two deflecting elements are disposed and a deflection mirror surface of each deflecting element deflects a light beam. For instance, in the optical scanning apparatus described in Japanese Patent Application Laid-Open Gazette No. S53-97447, a transmission optical system guides a light beam deflected by a first deflection mirror surface to a second deflection mirror surface, the second deflection mirror surface further deflects the light beam coming from the first deflection mirror surface, and the deflection angle of the light beam accordingly increases. The light beam emitted from the second deflection mirror surface is guided upon a surface-to-be-scanned through a scanning lens. Thus, the combination of the first and the second deflection mirror surfaces and the transmission optical system increases the deflection angle of the light beam.
Further, where vibration mirrors such as galvano-mirrors are used as the deflecting elements in the optical scanning apparatus above, a general approach to ensure vibrations at the maximum amplitude with minimum electric power is to apply a drive signal whose frequency matches with the resonance frequency of the vibration mirrors and to make the vibration mirrors resonate. As the vibration mirrors resonate, the vibration amplitude of the vibration mirrors is enhanced and the deflection angle of the light beam is increased.